The electroseismic (ES) method is an exploration tool designed to image conversions between electromagnetic and seismic energy. An electric current is created in the subsurface of the Earth by applying an electrical potential between two or more electrodes in contact with the Earth. These electrodes may be wires buried in trenches, pipes or rods placed in holes, casings of wells, either water wells or wells used in hydrocarbon exploration and production, or sheets of metal buried near the surface. The Earth current that is produced by these electrodes interacts with subterranean formations to create seismic waves. These seismic waves have particularly large amplitudes when they are created at the boundaries between rock containing hydrocarbon and non-reservoir rock.
To be effective, this method must distinguish seismic signals that originate at or near the Earth's surface from those generated at greater depth, particularly, signals originating at hydrocarbon reservoirs or other deep targets of interest. The present inventors have discovered numerous sources of unwanted seismic noises that can be generated near the surface electrodes, including:                ES conversions at a shallow water table or at other inhomogeneities in near-surface rock or soil;        Electric field excitation of buried pipes, fences, or other infrastructure;        ES conversions that occur at the boundary between the electrode and the soil; and        Seismic waves generated by forces between neighboring electrical conductors carrying electrical current.That these noise sources can be quite significant in magnitude is not obvious, but it has been discovered to be the case by the present inventors in the course of their experimentation with electroseismic prospecting. Such field experiments led to the realization that electroseismic signals originating near the electrodes can be larger than signals from greater depth because the electric field strength is typically large near the electrodes. It is thus desirable to develop methods of data collection that will distinguish between signals generated at depth and those generated at the surface, or will reduce the amplitude of surface noise, or will provide other means for minimizing the effects of surface noise.        
There is no current technology for suppression of near-surface noise in ES methods because the ES method is still relatively new and unutilized, dating back to the 1999 U.S. Pat. No. 5,877,995 to Thompson and Gist. Drawings in the Thompson and Gist patent and in U.S. Pat. No. 6,477,113 to Hornbostel and Thompson and WPO International Publication No. WO 02/091020 by Hornbostel, et al., show one set of two electrodes, and test data shown in those publications were obtained with this basic configuration. This existing technology permits neither the separation of shallow and deep signals nor mitigation of surface noise interference. The present invention satisfies this need.